Ken knew that Rokuro's death would be met with joy, but he wanted the Demon Lord's death as well as his death.
So he used his mind transfer ability to locate the Demon Lord and learned about the Demon Lord's past.
The Demon Lord was created by a demon centuries ago and sneaked into people's hearts to persuade them to do evil.
Ken located the Demon Lord and journeyed to his place. During the journey, Ken encountered several of the Demon Lord's right-hand men and easily defeated them. In fact, Rokuro was the strongest right-hand man.
Finally, Ken arrived at the Demon Lord's lair. At that moment, Ken's father appears and advances towards the Demon Lord. The Demon Lord shoots a magical arrow at Ken's father, but Ken slows down the movement of the arrow by cutting time and protects his father, telling him to retreat.
The Demon Lord launched a magical attack at Ken, and Ken tried to repel it using his own magic. Ken broke the Demon Lord's spell, making him miserable, and the Demon Lord had found the Demon Lord's weak points and knocked him to the ground.
The demons began to flee in panic at the Demon Lord's defeat. Ken advanced towards the Demon Lord and aimed to finish him off.
With the destruction of Ken, the Demon Lord, the world was saved once again.
Now, Ken was known as the most powerful wizard in the world and would continue to fight Demons in the future.
However, thanks to Ken's mind transfer ability, he also learned about possible bad events that would happen in the future. Therefore, Ken would continue to work to prevent possible future dangers.